Abstract

OBJECTIVES

We sought to evaluate the utility of excluding myocardial infarction (MI) in patients presenting to the emergency department (ED) with atrial fibrillation (AF) and to identify predictors of MI in this group.

BACKGROUND

Patients with AF are frequently admitted to the hospital, in part, to exclude an associated MI. There are no prospective data on unselected patients to support this common practice.

METHODS

We conducted a prospective cohort study of all patients who presented to a single-center ED with the primary diagnosis of AF.

RESULTS

Of a total of 255 patients, 190 (75%) were admitted to the hospital, and 109 of them underwent a standard “rule-out MI” protocol. Of these 109 patients, six (5.5%) were identified as having an acute MI at the time of admission. Chest pain was present in 39% of patients, with a sensitivity and specificity for the occurrence of MI of 100% and 65%, respectively. ST segment elevation or depression was present in 43% of patients, with a sensitivity and specificity of 100% and 51%. The presence of either major ST segment depression (>2 mm) or elevation on the admission electrocardiogram (ECG) was present in 6%, with a sensitivity of 100% and a specificity of 99%. The resulting positive and negative predictive values were 86% (95% confidence interval [CI] 42% to 99%) and 100% (95% CI 96% to 100%), respectively. Use of this criterion would have reduced the number of rule-out MIs in our study group by 94%, with no loss of sensitivity.

CONCLUSIONS

Chest pain and ST segment depression are extremely common findings in patients presenting to the ED with AF and have limited power to predict MI. In contrast, ECG evidence of ST segment elevation or depression >2 mm appears to be a reliable discriminator of which patients are at risk for MI. Patients without significant ST segment changes are at very low risk for MI and may not require performance of the rule-out MI protocol or hospital admission if clinically stable.

Atrial fibrillation (AF) is the most common arrhythmia diagnosis in the U.S. population. It is currently present in nearly 2.2 million Americans, and its prevalence is expected to increase with the aging population in the coming decades (1). Atrial fibrillation occurring as a complication of myocardial infarction (MI) is associated with a poor prognosis (2,3). Although the importance of AF as a primary marker for MI is uncertain, it is common practice for patients with AF to be admitted to the hospital for evaluation of a possible MI. The goal of the current study was to determine the appropriateness of this practice.

Methods

Patient group

Between January 1998 and January 1999, we prospectively identified and followed all patients with AF who presented to the emergency department (ED) of the Beth Israel Deaconess Medical Center. Demographic and clinical data were collected prospectively by a trained research nurse using standardized case report forms. Additional information collected included treatments performed in the ED (antiarrhythmic therapy, cardioversion), patient disposition (admit, discharge, indication) and clinical course once admitted.

During the study period, 255 patients presented to the ED with a primary diagnosis of new onset, recurrent (i.e., paroxysmal) or chronic AF (Fig. 1). Sixty-five patients were discharged directly from the ED, and 190 patients were admitted to the hospital. Of these 190 patients, 109 (57%) underwent a standard “rule-out MI” protocol at the discretion of the primary treating physician. These patients formed the primary analytic cohort for this study.

Diagram of consecutive patients who presented to the ED with AF over a one-year period.

Classification of symptoms

Symptoms, including chest pain, dyspnea and palpitations, were recorded as described by the patient to the ED physician. Because of difficulties in defining and enforcing standard definitions, we did not attempt to further specify the quality or severity of these symptoms.

Electrocardiographic classification

All 12-lead electrocardiograms (ECGs) were independently analyzed by two cardiologists (M.E.J. and P.Z.). The extent of ST segment elevation or depression was measured (to the nearest millimeter) in the leads with maximal depression or elevation. ST segment changes were measured at the maximal point of depression or elevation beginning at 80 ms after the QRS complex. To maximize the sensitivity of the ECG for the detection of myocardial ischemia, we did not attempt to differentiate potentially nonspecific ST segment depressions or depressions associated with left ventricular hypertrophy from more “ischemic-appearing” ST segment depressions. We prospectively defined major ST segment changes as any ST segment elevation or depression >2 mm. Minor ST segment changes were defined as ST segment depression ≤2 mm. Left ventricular hypertrophy was defined according to the criteria of Romhilt and Estes (4).

Determination of MI

The standard rule-out MI protocol included three serial creatine phosphokinase, MB isoenzyme (CPK-MB) determinations during a minimal 24-h admission to a medical floor with full disclosure telemetery. A patient was considered to have experienced a MI if any CPK-MB measurement was greater than two times the upper limit of normal (i.e., >20 IU/ml) (5).

Statistical analysis

Continuous data are described as the mean value ± SD, and categoric data are described as frequencies. Confidence limits were calculated using exact techniques. The association between the diagnosis of MI and specific clinical or electrophysiologic characteristics was evaluated using the t test for continuous variables or the Fisher exact test for categoric variables. A p value <0.05 was considered statistically significant.

Results

The total patient group consisted of the 255 consecutive patients who presented to the ED of the Beth Israel Deaconess Medical Center with a primary diagnosis of AF. The baseline characteristics of the total group, as well as of the subgroups (those who did and did not undergo the rule-out MI protocol) admitted to the hospital, are shown in Table 1. Compared with the patients who were either not admitted or admitted with no evaluation for MI, those patients in the rule-out MI group were older and more likely to be male and to carry a diagnosis of hypertension. The rule-out MI group was more likely to present with symptoms of chest pain or shortness of breath, but less likely to complain of palpitations. In the rule-out MI group, both new-onset and recurrent paroxysmal AF were common; no patients with chronic AF underwent evaluation for MI. Approximately one-half of the rule-out MI group had some ST segment depression or elevation on the admission ECG, but only 7% had major ST segment abnormalities.

Characteristics of the Total Study Group and Subgroups That Did and Did Not Undergo the Rule-Out Myocardial Infarction Protocollegend

Among the rule-out MI group, the primary physician listed a single reason for hospital admission (i.e., MI evaluation) in 30%, but two or more reasons in 70% of cases. The most common other indications for hospital admission in this group included rate control (48%), cardioversion (31%) and initiation of anticoagulation (39%).

Table 2compares the clinical and ECG features of those with and without MI in the subgroup that underwent the rule-out MI protocol (n = 109). The only clinical factors associated with MI were the presence of chest pain on presentation and new-onset AF. Other factors, including age, history of coronary artery disease and symptoms such as palpitations or shortness of breath at presentation, were not useful predictors of the occurrence of MI in our analytic cohort. Electrocardiographic factors predictive of MI included the presence of ST segment elevation or >2 mm of ST segment depression.

Association of Clinical and Electrocardiographic Factors With the Occurrence of Myocardial Infarctionlegend legend

The performance characteristics of various clinical and ECG criteria for distinguishing patients with and without MI are summarized in Table 3. The presence of major ST segment abnormalities on the presenting ECG (defined as >2 mm of ST segment depression or any ST segment elevation) was the best predictor of a subsequent MI, with nearly perfect sensitivity and specificity and a positive predictive value of 86% (95% confidence interval [CI] 42% to 99%) and a negative predictive value of 100% (95% CI 96% to 100%). The other potential criteria (new-onset AF, chest pain on presentation, any ST segment abnormalities) were also highly sensitive predictors of a subsequent MI, but had only modest discriminatory power owing to their limited specificities and resulting low positive predictive values (95% CI 11% to 14%).

The remaining 146 patients who did not undergo the rule-out MI protocol were followed clinically for subsequent complications, including possible myocardial ischemia or infarction. There were no instances of readmission for myocardial ischemia or infarction, cardiac catheterization or revascularization procedures performed in the month after their initial presentation. If, on the basis of their benign clinical courses, these patients were assumed not to have suffered a clinically significant MI at the time of presentation, the discriminatory power of major ST segment changes on the admitting ECG was unchanged, with a positive predictive value of 86% (95% CI 42% to 99%) and a negative predictive value of 100% (95% CI 98% to 100%).

Discussion

Incidence of MI

This study represents a comprehensive evaluation of all patients with AF who presented to a large urban hospital over a one-year period. In the subgroup of patients who underwent the rule-out MI protocol, we found that the incidence of MI at the time of admission was relatively low (6 [5.5%] of 109 patients). If those patients who were discharged or did not undergo systematic monitoring of cardiac isoenzymes, but had a benign subsequent clinical course, were also considered to have “ruled-out” for MI, the incidence of clinically significant MI in our AF group was only 2.3% (6 of 255 patients).

The relatively low incidence of MI in this patient group is not surprising. Previous studies have reported the incidence of MI in patients with new-onset AF to be 11% (6). In our population-based series, the incidence of documented MI was even lower, in part because only 69% of patients who underwent evaluation for MI had either chest pain or some evidence of myocardial ischemia on their admission ECG. This reflects the common practice in our institution (and many others) of admitting patients with AF to monitor for possible MI, even in the absence of any supporting signs or symptoms. Our study does not support this practice, however. Among the 31% of patients without chest pain or ST segment deviation on their admission ECG, there were no confirmed MIs.

Predictors of MI

Despite the low incidence of MI in our group, we identified several objective clinical and ECG features whose absence could be helpful in reliably identifying a subgroup of patients at extremely low risk of MI. These features include new-onset AF, the presence of chest pain and the presence of ST segment depression or elevation on the admission ECG. Although each of these factors was sensitive for the occurrence of MI, their value as potential discriminating factors for deciding which patients to admit and rule out MI and which to consider for less intensive management was limited by their low specificities (50% to 65%), and thus low positive predictive values (11% to 14%). In particular, the value of chest pain as a predictor of the occurrence of MI in this patient group was poor, with a positive predictive value of only 14% (95% CI 5% to 29%). This finding is consistent with previous studies of AF in which chest pain was a relatively common but nonspecific finding (7,8).

The most sensitive and specific predictor of MI was the presence of major ST segment deviation—defined as >2 mm of ST segment depression or any ST segment elevation—on the presenting ECG. In our patient group, this single factor was an excellent discriminator of patients with and without MI, with a positive predictive value of 86% (95% CI 42% to 99%) and a negative predictive value of 100% (95% CI 96% to 100%). This finding suggests that the vast majority of patients admitted to the hospital for evaluation of a possible MI might be managed equally well in a less intensive setting—either on an unmonitored medical unit or in the outpatient setting. In fact, use of this criterion would have reduced the number of MI evaluations in our group by 94%, with no loss of sensitivity. Even the adoption of less stringent ECG criteria, such as the presence of any degree of ST segment deviation, would have identified all of the patients with an acute MI in our group, while allowing for 46% to be managed more conservatively.

Although evaluation for a possible MI was cited frequently by the primary physician as an indication for hospital admission in our AF group, it is uncertain whether better risk stratification would substantially alter this practice and the high rate of hospital admission for these patients. In fact, most patients in our rule-out MI group had more than one indication for hospital admission, and evaluation for ongoing myocardial ischemia or infarction was cited as the sole indication for admission in only 21%. Nonetheless, many of the other indications for admission, including rate control, cardioversion and initiation of anticoagulation, can be safely performed on an outpatient basis with appropriate patient education, close follow-up and ancillary support (9). In contrast, ongoing myocardial ischemia or infarction is generally an “absolute” indication for hospital admission and close observation (10). Thus, we believe that more precise identification of patients with AF who are at risk for MI could substantially reduce the rate of hospital admission without affecting overall outcomes or quality of care.

Study limitations

Our study has several important limitations. The major limitation of our study is that the diagnostic thresholds we identified for prediction of MI among patients with AF were selected on a post-hoc basis. Accordingly, our findings should be viewed as hypothesis-generating and will require validation in an independent data set before their widespread implementation in clinical practice. Second, given the limited number of MIs in our patient group, the confidence limits for our estimates of sensitivity (and positive predictive value) are wide. Nonetheless, the major role of our diagnostic criteria is in identifying patients at low risk for MI who do not necessarily require hospital admission if otherwise stable. The value of these criteria is thus related mainly to their negative predictive values, for which the confidence limits are substantially more narrow. Finally, the fact that our group was drawn from a single, urban, academic medical center may also limit the generalizability of our findings to other settings and patient groups.

Conclusions

Despite the common practice of admitting patients with AF to the hospital to “rule out MI,” the incidence of MI in such patients without other evidence of myocardial ischemia is low. The presence of major ST segment abnormalities on the presenting ECG was both a sensitive and specific predictor of MI in our patients. Use of this criterion (or other related factors) must be prospectively validated, but may avoid unnecessary testing and hospital admissions and result in important cost savings without loss of quality of care.

Footnotes

☆ Dr. Cohen is supported in part by a Clinician-Scientist award from the American Heart Association.

Abbreviations

AF

atrial fibrillation

CI

confidence interval

CPK-MB

creatine phosphokinase, MB isoenzyme

ECG

electrocardiogram or electrocardiographic

ED

emergency department

MI

myocardial infarction

Received December 27, 1999.

Revision received March 21, 2000.

Accepted May 1, 2000.

American College of Cardiology

References

(1997) Management of patients with atrial fibrillation: a statement for healthcare professionals from the Subcommittee on Electrocardiography and Electrophysiology, American Heart Association. Circulation93:1262–1277.

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